Electrode plate for nonaqueous electrolyte secondary battery, nonaqueous electrolyte secondary battery including the same, and method for manufacturing the same

ABSTRACT

An electrode active material layer containing an electrode active material and a binder. The electrode active material layer includes a portion containing the smallest amount of the binder in a middle region across the thickness of the electrode active material layer. The binder is distributed in the electrode active material layer such that the amount of the binder increases continuously from the portion toward the core and an outer surface of the electrode active material layer. Preferably, the amount of the binder present in the electrode active material layer per unit thickness is limited to more than 10 in a region extending 10% of the thickness from the outer surface of the electrode active material layer, with 10 being assigned to the amount of the binder present in the electrode active material layer per unit thickness if the binder is uniformly distributed.

TECHNICAL FIELD

The present invention relates to improvements in the cyclecharacteristics of nonaqueous electrolyte secondary batteries.

BACKGROUND ART

Nonaqueous electrolyte secondary batteries are widely used as powersupplies for driving portable devices such as cellular phones, smartphones, and notebook PCs because of their high energy density and highcapacity. Recently, there has been a need for further improvements inthe cycle characteristics of nonaqueous electrolyte secondary batteries.

Electrode plates used in nonaqueous electrolyte secondary batteries aremanufactured by applying to a core an electrode active material slurrycontaining an electrode active material, a binder, and a solvent, dryingthe resulting coating, and pressing the coated core. This method isadvantageous in terms of productivity and discharge characteristics.

The use of an excessive amount of binder, which does not directlycontribute to charge and discharge, decreases the discharge capacity.The use of an insufficient amount of binder decreases the peelingstrength of the electrode active material layer and thus results in theelectrode active material easily peeling off the core.

Example techniques related to binders for nonaqueous electrolytesecondary batteries are disclosed in PTLs 1 to 5 below.

CITATION LIST Patent Literature

PTL 1: Japanese Published Unexamined Patent Application No. 5-89871

PTL 2: Japanese Published Unexamined Patent Application No. 2011-192539

PTL 3: Japanese Published Unexamined Patent Application No. 9-147834

PTL 4: Japanese Published Unexamined Patent Application No. 2008-258055

PTL 5: Japanese Published Unexamined Patent Application No. 10-284059

PTL 1 discloses a secondary battery electrode including a coating layerof an active material on a metal current collector. A binder isdistributed in the active material layer with a binder distributioncoefficient of 0.5 to 5.0. PTL 1 teaches that this technique providesgood battery characteristics.

PTL 2 discloses an electrode including a current collector, a firstmixture layer formed on the current collector, and a second mixturelayer formed on the first mixture layer. The second mixture layercontains a smaller amount of binder than the first mixture layer. PTL 2teaches that this technique provides an electrode for nonaqueouselectrolyte secondary batteries with good rate characteristics and cyclecharacteristics and high capacity.

PTL 3 discloses a negative electrode active material layer composed offirst, second, and third negative electrode active material layers 7 a,7 b, and 7 c. The amount of the binder present in the negative electrodeactive material layer decreases stepwise outward from an interface witha current collector. PTL 3 teaches that this technique provides abattery with high capacity and energy density.

PTL 4 discloses a positive electrode including a positive electrodecurrent collector and a plurality of positive electrode mixture layersdisposed on the positive electrode current collector and containing apositive electrode material powder and a positive electrode binder. Ofthe plurality of positive electrode mixture layers, the innermostpositive electrode mixture layer, i.e., the layer closest to thepositive electrode current collector, has a higher positive electrodebinder content than the other positive electrode mixture layers andcontains 4% to 7% by weight of positive electrode binder. PTL 4 teachesthat this technique provides a secondary battery with good outputcharacteristics and cycle life characteristics.

PTL 5 discloses a technique for forming a negative electrode materiallayer on a current collector. This technique includes applying a bindersolution containing a fluoropolymer binder and an organic solvent toeach surface of a current collector made of copper foil, semidrying thecoating to form a semidried binder layer, applying a carbon-basedmaterial dispersion prepared by mixing a mixture of graphite and afluoropolymer with an organic solvent to the semidried binder layer oneach side of the current collector, and drying the coating to form anegative electrode material layer. PTL 5 teaches that this techniqueprovides sufficient adhesion between the negative electrode materiallayer and the negative electrode current collector without decreasingthe discharge capacity.

Unfortunately, the batteries based on the techniques disclosed in PTLs 1to 5 have insufficient cycle characteristics.

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide a nonaqueouselectrolyte secondary battery with good cycle characteristics.

Solution to Problem

To solve the foregoing problem, an electrode plate for a nonaqueouselectrolyte secondary battery according to the present invention isconfigured as follows.

The electrode plate for a nonaqueous electrolyte secondary batteryincludes a core and an electrode active material layer disposed thereon.The electrode active material layer contains an electrode activematerial and a binder. The electrode active material layer includes aportion containing the smallest amount of the binder in a middle regionacross the thickness of the electrode active material layer. The binderis distributed in the electrode active material layer such that theamount of the binder increases continuously from the portion toward thecore and an outer surface of the electrode active material layer.

After conducting research on the relationship between the binder and thecycle characteristics, the inventors have made the following findings.As the binder is present in a larger amount, the electrode activematerial can accept fewer lithium ions since the surface thereof iscovered by the binder, whereas the electrode active material layer canretain a larger amount of nonaqueous electrolyte. The cyclecharacteristics can be improved by achieving a balance of the ability toaccept lithium ions and the amount of nonaqueous electrolyte duringcharge-discharge cycles.

In the above configuration according to the present invention, a largeramount of the binder is present near the core, where the amount ofnonaqueous electrolyte tends to be insufficient, and accordingly alarger amount of nonaqueous electrolyte is retained near the core. Thenonaqueous electrolyte is also supplied from near the outer surface,where a larger amount of binder is present and accordingly a largeramount of nonaqueous electrolyte is retained, to the middle region,where a smaller amount of binder is present (i.e., a smaller amount ofnonaqueous electrolyte is retained) during charge-discharge cycles. As aresult, the amount of nonaqueous electrolyte is balanced throughout theelectrode active material layer, and charge-discharge cycles proceedsmoothly throughout the electrode active material layer. This improvesthe cycle characteristics and reduces the resistance of the electrodeplate. The abundant supply of nonaqueous electrolyte in the aboveconfiguration negates the adverse effect of the decrease in the abilityto accept lithium ions, thus providing a nonaqueous electrolytesecondary battery with good cycle characteristics. The binder, which ispresent in a larger amount near the core, also reduces peeling of theelectrode active material layer off the core, for example, upon impact.

The middle region of the electrode active material layer refers to aregion extending from a position 10% of the thickness of the electrodeactive material layer to a position 90% of the thickness of theelectrode active material layer from the surface of the electrode activematerial layer facing the core. Preferably, the portion containing thesmallest amount of the binder is located in a region extending from aposition 20% of the thickness of the electrode active material layer toa position 80% of the thickness of the electrode active material layerfrom the surface of the electrode active material layer facing the core.More preferably, the portion containing the smallest amount of thebinder is located in a region extending from a position 45% of thethickness of the electrode active material layer to a position 55% ofthe thickness of the electrode active material layer from the surface ofthe electrode active material layer facing the core.

The binder is distributed such that the amount of the binder changescontinuously, and there is no interface where the amount of the binderper unit thickness changes discontinuously in the electrode activematerial layer. Preferably, the gradient of the binder concentration issmaller in the middle region than near the outer surface and the core.

In the above configuration, the amount of the binder present in theelectrode active material layer per unit thickness may be more than 10throughout a region extending from a position 90% of the thickness ofthe electrode active material layer to a position 100% of the thicknessof the electrode active material layer from the surface of the electrodeactive material layer facing the core, with 10 being assigned to theamount of the binder present in the electrode active material layer perunit thickness if the binder is uniformly distributed.

The above limitation is preferred to further improve the ability toretain the nonaqueous electrolyte near the outer surface.

The term “amount of binder present in the electrode active materiallayer per unit thickness” does not refer to the amount of the binderpresent in a particular region across the thickness of the electrodeactive material layer; rather, it refers to the amount of the binderpresent across the thickness of the electrode active material layer asexpressed differentially.

By “the amount of the binder present in the electrode active materiallayer per unit thickness is more than 10 in a region extending from aposition 90% of the thickness of the electrode active material layer toa position 100% of the thickness of the electrode active material layerfrom the surface of the electrode active material layer facing the core,with 10 being assigned to the amount of the binder present in theelectrode active material layer per unit thickness if the binder isuniformly distributed”, it is meant that there is no portion where thebinder is present in an amount of 10 or less in the region extendingfrom the position 90% of the thickness of the electrode active materiallayer to the position 100% of the thickness of the electrode activematerial layer from the surface of the electrode active material layerfacing the core (i.e., in the region extending 10% of the thickness fromthe outer surface).

In the above configuration, the amount of the binder present in theelectrode active material layer per unit thickness may be less than 10in a region extending from a position 45% of the thickness of theelectrode active material layer to a position 55% of the thickness ofthe electrode active material layer from the surface of the electrodeactive material layer facing the core, with 10 being assigned to theamount of the binder present in the electrode active material layer perunit thickness if the binder is uniformly distributed in the electrodeactive material layer. The amount of the binder present in the electrodeactive material layer per unit thickness may be more than 10 in a regionextending from a position 0% of the thickness of the electrode activematerial layer to a position 10% of the thickness of the electrodeactive material layer from the surface of the electrode active materiallayer facing the core.

The above limitation on the amount of the binder in the region extending10% of the thickness of the electrode active material layer from thesurface of the electrode active material layer facing the core (i.e.,the region extending from the position 0% of the thickness of theelectrode active material layer to the position 10% of the thickness ofthe electrode active material layer from the surface of the electrodeactive material layer facing the core) is preferred to further reducepeeling of the electrode active material layer off the core. The abovelimitation on the amount of the binder in the middle region (i.e., theregion extending from the position 45% of the thickness of the electrodeactive material layer to the position 55% of the thickness of theelectrode active material layer from the surface of the electrode activematerial layer facing the core) is preferred to further improve thecycle characteristics.

Preferably, the lower limit of the amount of the binder in the regionextending from the position 45% of the thickness of the electrode activematerial layer to the position 55% of the thickness of the electrodeactive material layer from the surface of the electrode active materiallayer facing the core is the minimum amount required to form theelectrode active material layer.

In the above configuration, the electrode active material present in theelectrode active material layer and the core may have a contact areafraction of 30% or more.

The above limitation is preferred to further reduce peeling of theelectrode active material layer off the core. Preferably, the activematerial particles are partially embedded in the core.

Examples of the binder include known binders such as polyvinylidenefluoride, polytetrafluoroethylene, and styrene-butadiene rubber. Inparticular, styrene-butadiene rubber and modified styrene-butadienerubbers are preferred since they can be used in combination with water,which is inexpensive and environmentally friendly, as a solvent to formthe electrode active material layer. Suitable examples of the modifiedstyrene-butadiene rubbers include carboxyl-modified and amino-modifiedstyrene-butadiene rubbers.

The content by mass of the binder in the electrode active material layermay vary depending on the type of the binder. For example, the contentby mass of the binder in the electrode active material layer may be 0.1%to 3.0% by mass for polyvinylidene fluoride, may be 0.1% to 5.0% by massfor polytetrafluoroethylene, or may be 0.1% to 5.0% by mass forstyrene-butadiene rubber and modified styrene-butadiene rubbers.

To solve the foregoing problem, a nonaqueous electrolyte secondarybattery according to the present invention is configured as follows.

The nonaqueous electrolyte secondary battery includes a positiveelectrode plate and a negative electrode plate. At least one of thepositive electrode plate and the negative electrode plate is any of theabove electrode plates for nonaqueous electrolyte secondary batteries.

To solve the foregoing problem, a method for manufacturing a nonaqueouselectrolyte secondary battery according to the present invention isconfigured as follows.

The method for manufacturing a nonaqueous electrolyte secondary batteryincludes a first applying step of applying to a core a first electrodeactive material slurry containing an electrode active material, abinder, and a solvent; a second applying step of applying a secondelectrode active material slurry containing an electrode activematerial, a binder, and a solvent to an undried layer of the firstelectrode active material slurry; and a drying step of evaporating thesolvent after the second step. The second electrode active materialslurry has a lower solvent content by mass than the first electrodeactive material slurry. The electrode active material layer includes aportion containing the smallest amount of the binder in a middle regionacross the thickness of the electrode active material layer. The binderis distributed in the electrode active material layer such that theamount of the binder increases continuously from the portion toward thecore and an outer surface of the electrode active material layer.

The first electrode active material slurry is applied to the core, andthe second electrode active material slurry, which has a lower solventcontent (i.e., a higher solids content (including the electrode activematerial, the binder, and other optional ingredients such asthickeners)) than the first electrode active material slurry, is appliedto the undried layer of the first electrode active material slurry. Thebinder sinks below the interface between the layers of the first andsecond electrode active material slurries. This inhibits rising of thebinder. In the subsequent drying step, the binder rises as the solventevaporates from the electrode active material layer. These actionsresult in an electrode having the binder distribution as describedabove.

The drying speed during the drying step is preferably 5 g(solvent)/m²·sor less, more preferably 4 g(solvent)/m²·s or less. An extremely highdrying speed may result in uneven drying and cracking of the electrodeactive material layer. The drying speed is also preferably 2g(solvent)/m²·s or more, more preferably 3 g(solvent)/m²·s or more. Anextremely low drying speed may result in low productivity andinsufficient rising of the binder. The drying speed can be adjusted, forexample, by controlling the atmospheric temperature and pressure duringthe drying step.

The first and second electrode active material slurries preferablycontain the same binder. The first and second electrode active materialslurries may have the same or different compositions. A third electrodeactive material layer may be formed on the undried second electrodeactive material layer.

The mass ratio of the solids in the first electrode active materialslurry to the solids in the second electrode active material slurrybefore the drying step is preferably 0.1 to 10, more preferably 0.5 to2.

Advantageous Effects of Invention

As described above, the present invention provides a nonaqueouselectrolyte secondary battery with good cycle characteristics.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a graph showing the binder distributions of negative electrodeactive material layers of Examples 1 and 2 and Comparative Example 1.

DESCRIPTION OF EMBODIMENTS

The best mode of practicing the present invention will now be describedin detail by way of the following examples. It should be understood thatthe following examples are not intended to limit the present invention;various modifications are possible without departing from the spiritthereof.

EXAMPLES Example 1 Fabrication of Positive Electrode

A positive electrode active material slurry was prepared by mixing 100parts by mass of lithium cobaltate (LiCoO₂), serving as a positiveelectrode active material, 2 parts by mass of acetylene black, servingas a conductor, 2 parts by mass of polyvinylidene fluoride (PVdF),serving as a binder, and N-methyl-2-pyrrolidone.

The positive electrode active material slurry was applied to eachsurface of an aluminum core with a thickness of 15 μm using a doctorblade and was dried. The coated core was then pressed to a thickness of170 μm using a press roller and was cut to form a positive electrodeplate.

Fabrication of Negative Electrode

A negative electrode active material slurry A (second electrode activematerial slurry) was prepared by mixing 100 parts by mass of artificialgraphite, serving as a negative electrode active material, 1 part bymass of carboxymethylcellulose, serving as a thickener, 1 part by mass,on a styrene-butadiene rubber (SBR) basis, of a dispersion ofstyrene-butadiene rubber in water (solids content: 40%), serving as abinder, and an appropriate amount of water that gave a total solidscontent of 48% to 55%, in a double-arm mixer.

A negative electrode active material slurry B (first electrode activematerial slurry) was prepared by mixing a portion of the negativeelectrode active material slurry A and an appropriate amount of waterthat gave a total solids content of 40% to 48% in a double-arm mixer.

The solids in the negative electrode active material slurry B had thesame composition as the solids in the negative electrode active materialslurry A. The negative electrode active material slurry B had a higherwater (solvent) content by mass than the negative electrode activematerial slurry A.

The negative electrode active material slurry B was applied to eachsurface of a copper core with a thickness of 10 μm using a doctor blade.The negative electrode active material slurry A was then applied to theundried layer of the negative electrode active material slurry B using adoctor blade. The solids content by mass of the layer of the negativeelectrode active material slurry A was twice the solids content by massof the layer of the negative electrode active material slurry B.

The coating was then dried at an average drying speed of 3g(solvent)/m²·s. The coated core was pressed to a thickness of 200 μmusing a press roller and was cut to form a negative electrode plate.

This negative electrode plate was analyzed for the distribution of thebinder (SBR) across the thickness of the active material layer by imageprocessing of a bromine-stained cross-section. FIG. 1 shows the binderdistribution across the thickness of the active material layer. In thisgraph, the binder distribution is expressed in relative values, with 10being assigned to the amount of the binder present in the negativeelectrode active material layer per unit thickness if the binder isuniformly distributed.

The amount of the binder present in the electrode active material layerper unit thickness was 12 to 15 throughout the region extending from aposition 90% of the thickness of the electrode active material layer toa position 100% of the thickness of the electrode active material layerfrom the surface of the electrode active material layer facing the core,with 10 being assigned to the amount of the binder present in thenegative electrode active material layer per unit thickness if thebinder is uniformly distributed. The amount of the binder present in theelectrode active material layer per unit thickness was 5.5 to 7throughout the region extending from a position 45% of the thickness ofthe electrode active material layer to a position 55% of the thicknessof the electrode active material layer from the surface of the electrodeactive material layer facing the core. The amount of the binder presentin the electrode active material layer per unit thickness was 12 to 15throughout the region extending from a position 0% of the thickness ofthe electrode active material layer to a position 10% of the thicknessof the electrode active material layer from the surface of the electrodeactive material layer facing the core. The contact area fraction of theactive material (artificial graphite) particles present in the activematerial layer and the core was 30% before pressing and was not lessthan 30% after pressing.

Fabrication of Electrode Assembly

A cylindrical electrode assembly was fabricated by winding the positiveand negative electrode plates with a polyethylene microporous separator(thickness: 20 μm) therebetween and attaching a tape to the outermostsurface thereof.

Preparation of Nonaqueous Electrolyte

A nonaqueous electrolyte was prepared by mixing ethylene carbonate,diethyl carbonate, and ethyl methyl carbonate in a volume ratio of80:5:15 (at 25° C. and 1 atmosphere), dissolving LiPF₆, serving as anelectrolyte salt, in a concentration of 1.0 M (mol/L), and adding 3% bymass of vinylene carbonate.

Assembly of Battery

The electrode assembly and 5.5 g of the nonaqueous electrolyte wereplaced in a cylindrical can. The opening of the can was then sealed witha sealing member to obtain a nonaqueous electrolyte secondary battery(height: 65 mm, diameter: 18 mm) of Example 1.

Example 2

A nonaqueous electrolyte secondary battery of Example 2 was fabricatedas in Example 1 except that the solids content by mass of the layer ofthe negative electrode active material slurry A was once the solidscontent by mass of the layer of the negative electrode active materialslurry B, and the coating was dried at an average drying speed of 4g(solvent)/m²·s (with the total solids content being the same as inExample 1).

This negative electrode plate was analyzed for the distribution of thebinder (SBR) across the thickness of the active material layer in thesame manner as described above. The amount of the binder present in theelectrode active material layer per unit thickness was 14 to 18throughout the region extending from a position 90% of the thickness ofthe electrode active material layer to a position 100% of the thicknessof the electrode active material layer from the surface of the electrodeactive material layer facing the core, with 10 being assigned to theamount of the binder present in the negative electrode active materiallayer per unit thickness if the binder is uniformly distributed. Theamount of the binder present in the electrode active material layer perunit thickness was 3 to 4.5 throughout the region extending from aposition 45% of the thickness of the electrode active material layer toa position 55% of the thickness of the electrode active material layerfrom the surface of the electrode active material layer facing the core.The amount of the binder present in the electrode active material layerper unit thickness was 16 to 21 throughout the region extending from aposition 0% of the thickness of the electrode active material layer to aposition 10% of the thickness of the electrode active material layerfrom the surface of the electrode active material layer facing the core.The contact area fraction of the active material (artificial graphite)particles present in the active material layer and the core was 30%before pressing and was not less than 30% after pressing.

Comparative Example 1

A nonaqueous electrolyte secondary battery of Comparative Example 1 wasfabricated as in Example 1 except that a negative electrode wasfabricated using only a negative electrode active material slurry Chaving a total solids content of 50%, and the coating was dried at anaverage drying speed of 4 g(solvent)/m²·s (with the total solids contentbeing the same as in Example 1).

This negative electrode plate was analyzed for the distribution of thebinder (SBR) across the thickness of the active material layer in thesame manner as described above. The binder was uniformly distributed(see FIG. 1). The contact area fraction of the active material(artificial graphite) particles present in the active material layer andthe core was 30% before pressing and was not less than 30% afterpressing.

Cycle Characteristics Test

Batteries fabricated as in Examples 1 and 2 and Comparative Example 1were charged and discharged twice under the following conditions andwere then stored in an isothermal bath at 45° C. for 7 days. Thesebatteries were charged and discharged again for 500 cycles under thefollowing conditions. The capacity retention was calculated by thefollowing equation. The results are shown in Table 1 below.

Charge: charged at a constant current of 1,400 mA to a voltage of 4.2 Vand then at a constant voltage of 4.2 V to a current of 100 mA, at 25°C.

Discharge: discharged at a constant current of 2,000 mA to a voltage of3.0 V, at 25° C.

Capacity retention (%)=discharge capacity at 500th cycle/dischargecapacity at 1st cycle×100

Low-Temperature Cycle Characteristics Test

Batteries fabricated as in Examples 1 and 2 and Comparative Example 1were charged and discharged for 100 cycles under the followingconditions. The capacity retention was calculated by the followingequation. The results are shown in Table 1 below.

Charge: charged at a constant current of 1,400 mA to a voltage of 4.2 Vand then at a constant voltage of 4.2 V to a current of 100 mA, at −5°C.

Discharge: discharged at a constant current of 2,000 mA to a voltage of3.0 V, at −5° C.

Capacity retention (%)=discharge capacity at 100th cycle/dischargecapacity at 1st cycle×100

Measurement of Internal Resistance

Batteries fabricated as in Examples 1 and 2 and Comparative Example 1were allowed to stand at 20° C. for 30 minutes and were then charged ata constant current of 0.2 It (440 mA) to a state of charge (SOC) of 50%.The batteries were then allowed to stand at 25° C. for 30 minutes. Thebatteries were then charged and discharged at a constant current of 1 It(2,200 mA) for 10 seconds. The voltage gradient dV/dA was calculated bydividing the difference between the charge-discharge cut-off voltage andthe open-circuit voltage at an SOC of 50% by the current that flowed. InTable 1 below, the results are expressed as relative values, with 100being assigned to Comparative Example 1.

TABLE 1 Binder distribution Capacity retention Near Near outer (%)Internal core Middle surface −5° C. 25° C. resistance Example 1 12-155.5-7   12-15 88 62 95 Example 2 16-21   3-4.5 14-18 90 73 91Comparative 10 10 10 83 56 100 Example 1

FIG. 1 shows the binder distributions of the negative electrode activematerial layers.

In Examples 1 and 2, as shown in Table 1 and FIG. 1, the amount of thebinder present in the negative electrode active material layer increasedfrom the portion containing the smallest amount of binder in the middleregion across the thickness of the electrode active material layertoward the core and the outer surface of the electrode active materiallayer. The batteries of Examples 1 and 2 had cycle capacity retentionsat −5° C. of 88% and 90%, cycle capacity retentions at 25° C. of 62% and73%, and internal resistances of 95 and 91, respectively. In ComparativeExample 1, the amount of the binder present in the negative electrodeactive material layer was constant. The battery of Comparative Example 1had a cycle capacity retention at −5° C. of 83%, a cycle capacityretention at 25° C. of 56%, and an internal resistance of 100. Theseresults show that the batteries of Examples 1 and 2 had bettercharacteristics than the battery of Comparative Example 1.

This can be explained as follows. As the binder is present in a largeramount, the electrode active material can accept fewer lithium ionssince the surface thereof is covered by the binder, whereas theelectrode active material layer can retain a larger amount of nonaqueouselectrolyte.

In Examples 1 and 2, the binder was distributed in the electrode activematerial layer such that the amount of the binder increased continuouslyfrom the portion containing the smallest amount of binder in the middleregion toward the core and the outer surface of the electrode activematerial layer. In this configuration, a larger amount of the binder ispresent near the core, where the amount of nonaqueous electrolyte tendsto be insufficient, and accordingly a larger amount of nonaqueouselectrolyte is retained near the core. The nonaqueous electrolyte isalso supplied from near the outer surface, where a larger amount of thebinder is present and accordingly a larger amount of nonaqueouselectrolyte is retained, to the middle region, where a smaller amount ofthe binder is present (i.e., a smaller amount of nonaqueous electrolyteis retained) during charge-discharge cycles. As a result, the amount ofnonaqueous electrolyte is balanced throughout the electrode activematerial layer, and charge-discharge cycles proceed smoothly throughoutthe electrode active material layer. This improves the cyclecharacteristics and reduces the resistance. The binder, which is presentin a larger amount near the core, also reduces peeling of the electrodeactive material layer off the core, for example, upon impact.

The results also show that the batteries of Examples 1 and 2 tended tohave a higher capacity retention and a lower internal resistance withincreasing gradient of the binder distribution. The results also showthat the gradient of the binder distribution can be controlled bychanging the drying speed and the mass ratio of the solids in one activematerial slurry to the solids in the other active material slurry.

Additional Items

Whereas the present invention is applied to negative electrode plates inthe foregoing examples, it can instead be applied to positive electrodeplates or to both electrode plates.

Examples of the binder that can be used in the present invention includestyrene-butadiene rubber, polyvinylidene fluoride, andpolytetrafluoroethylene. The binder may be selected depending on, forexample, the type of active material and the type of solvent.

A wide variety of known electrode active materials can be used in theelectrode plates according to the present invention. Examples of thepositive electrode active material include lithium cobalt nickelmanganese oxide (Li_(x)Ni_(a)Mn_(b)Co_(c)O₂, where 0.9<x≦1.2 anda+b+c=1), spinel-type lithium manganate (Li_(x)Mn₂O₄), and those inwhich the transition metal elements are replaced by other elements.These materials can be used alone or in mixture.

Examples of the negative electrode active material include carbon-basedmaterials capable of absorbing and desorbing lithium ions (e.g.,graphite, acetylene black, carbon black, and amorphous carbon),silicon-based materials, metallic lithium, lithium alloys, and metaloxides capable of absorbing and desorbing lithium ions. These materialscan be used alone or in mixture.

The solvent used for slurry preparation may be water forstyrene-butadiene rubber or may be N-methyl-2-pyrrolidone forpolyvinylidene fluoride and polytetrafluoroethylene.

Examples of the nonaqueous solvent for nonaqueous electrolytes includecarbonates, lactones, ketones, ethers, and esters. Specific examplesinclude ethylene carbonate, propylene carbonate, butylene carbonate,diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate,γ-butyrolactone, γ-valerolactone, γ-dimethoxyethane, tetrahydrofuran,and 1,4-dioxane.

Examples of the electrolyte salt other than LiPF₆ for nonaqueouselectrolytes include LiBF₄, LiAsF₆, LiCF₃SO₃, LiN(CF₃SO₂)₂,LiN(C₂F₅SO₂)₂, and LiClO₄, which can be used alone or in mixture. Theelectrolyte salt is preferably dissolved in the nonaqueous solvent in aconcentration of 0.5 to 2.0 mol/L.

Examples of the core for positive electrodes include pure aluminum coresand aluminum alloy cores. Examples of the core for negative electrodesinclude pure copper cores and copper alloy cores.

INDUSTRIAL APPLICABILITY

As described above, the present invention provides a nonaqueouselectrolyte secondary battery with good cycle characteristics; thus, ithas wide industrial applicability.

1. An electrode plate for a nonaqueous electrolyte secondary battery,comprising a core and an electrode active material layer disposedthereon, the electrode active material layer containing an electrodeactive material and a binder, wherein the electrode active materiallayer includes a portion containing the smallest amount of the binder ina middle region across the thickness of the electrode active materiallayer, the binder being distributed in the electrode active materiallayer such that the amount of the binder increases continuously from theportion toward the core and an outer surface of the electrode activematerial layer.
 2. The electrode plate for a nonaqueous electrolytesecondary battery according to claim 1, wherein the amount of the binderpresent in the electrode active material layer per unit thickness ismore than 10 in a region extending from a position 90% of the thicknessof the electrode active material layer to a position 100% of thethickness of the electrode active material layer from a surface of theelectrode active material layer facing the core, with 10 being assignedto the amount of the binder present in the electrode active materiallayer per unit thickness if the binder is uniformly distributed.
 3. Theelectrode plate for a nonaqueous electrolyte secondary battery accordingto claim 2, wherein the amount of the binder present in the electrodeactive material layer per unit thickness is less than 10 in a regionextending from a position 45% of the thickness of the electrode activematerial layer to a position 55% of the thickness of the electrodeactive material layer from the surface of the electrode active materiallayer facing the core, and is more than 10 in a region extending from aposition 0% of the thickness of the electrode active material layer to aposition 10% of the thickness of the electrode active material layerfrom the surface of the electrode active material layer facing the core,with 10 being assigned to the amount of the binder present in theelectrode active material layer per unit thickness if the binder isuniformly distributed in the electrode active material layer.
 4. Theelectrode plate for a nonaqueous electrolyte secondary battery accordingto claim 1, wherein the electrode active material present in theelectrode active material layer and the core have a contact areafraction of 30% or more.
 5. The electrode plate for a nonaqueouselectrolyte secondary battery according to claim 1, wherein the binderis styrene-butadiene rubber and/or a modified styrene-butadiene rubber.6. A nonaqueous electrolyte secondary battery comprising a positiveelectrode plate and a negative electrode plate, at least one of thepositive electrode plate and the negative electrode plate being theelectrode plate for a nonaqueous electrolyte secondary battery accordingto claim
 1. 7. A method for manufacturing a nonaqueous electrolytesecondary battery, comprising: a first applying step of applying to acore a first electrode active material slurry comprising an electrodeactive material, a binder, and a solvent; a second applying step ofapplying a second electrode active material slurry comprising anelectrode active material, a binder, and a solvent to an undried layerof the first electrode active material slurry, the second electrodeactive material slurry having a lower solvent content by mass than thefirst electrode active material slurry; and a drying step of evaporatingthe solvent after the second step, wherein the electrode active materiallayer includes a portion containing the smallest amount of the binder ina middle region across the thickness of the electrode active materiallayer, and the binder is distributed in the electrode active materiallayer such that the amount of the binder increases continuously from theportion toward the core and an outer surface of the electrode activematerial layer.
 8. A nonaqueous electrolyte secondary battery comprisinga positive electrode plate and a negative electrode plate, at least oneof the positive electrode plate and the negative electrode plate beingthe electrode plate for a nonaqueous electrolyte secondary batteryaccording to claim
 2. 9. A nonaqueous electrolyte secondary batterycomprising a positive electrode plate and a negative electrode plate, atleast one of the positive electrode plate and the negative electrodeplate being the electrode plate for a nonaqueous electrolyte secondarybattery according to claim
 3. 10. A nonaqueous electrolyte secondarybattery comprising a positive electrode plate and a negative electrodeplate, at least one of the positive electrode plate and the negativeelectrode plate being the electrode plate for a nonaqueous electrolytesecondary battery according to claim 4.